1. Field of the Invention
The present invention regards a driving circuit for piezoelectric actuators, in particular for a read/write transducer for hard disks.
2. Description of the related art
As is known, piezoelectric actuators are currently used in actuator device for hard disks provided with a dual actuation stage, in which a first actuation stage carries out rough displacement of a read/write transducer for a hard disk during track following, and a second actuation stage, constituted by the piezoelectric actuators, performs finer adjustment of the position of the read/write transducer.
Piezoelectric actuators for displacement of readwrite transducers for hard disks are mainly used for the following reasons:
1. an increasingly higher data-storage capacity is required inside the hard disk (understood as Gbytes per disk) resulting in a continuous increase in the density of data stored per unit area, and hence in a continuously decreasing distance between the tracks. It follows that, in order to have correct reading/writing of the hard disk, it is necessary to position the readwrite transducer with a high degree of precision. Given that piezoelectric actuators are positioned close to the transducer, they are able to control submicrometric displacements, thus ensuring the high precision required; and
2. an increasingly higher speed of access to the data stored on the hard disk is required, which finds its limitation precisely in the actuator device. Given that piezoelectric actuators have to actuate very small masses, they are able to position the read/write transducer in a considerably short time as compared to single-actuation actuator devices.
An example of an actuator device for a hard disk with dual actuation stage is schematically illustrated in FIG. 1. The actuator device 1 for a hard disk comprises an induction motor 2 (also referred to as xe2x80x9cvoice coil motorxe2x80x9d) to which a suspension 3 consisting of a lamina is fixed in cantilever fashion. The suspension 3 terminates with a flexible portion 4 which carries, at its free end, a read/write transducer 5 (referred to as xe2x80x9csliderxe2x80x9d), set, in the operating condition, facing a surface of a hard disk 6, and a pair of actuators 7a and 7b of piezoelectric material, each set on one side of the free end of the flexible portion 4. In particular, each piezoelectric actuator 7a, 7b consists of a chip of piezoelectric material enclosed between two metal plates forming the two electrodes of the actuator. One electrode is connected to the flexible portion 4 (ground), whilst the other is floating.
Advantageously, the readwrite transducer 5 is fixed to the flexible portion 4 by means of a joint 8 (referred to as xe2x80x9cgimbalxe2x80x9d). In addition, the read/write transducer 5 supports a read/write head 9 constituting the read/write device proper.
In the actuator device 1, the first actuation stage consists of the induction motor 2 displacing the ensemble formed by the suspension 3 and the read/write transducer 5 through the hard disk during track search (rough displacement), whereas the second actuation stage 10 comprises the flexible portion 4 and the piezoelectric actuators 7a, 7b, carrying out fine control of the position of the read/write transducer 5, following the track (finer regulation).
In particular, the flexible portion 4 converts the mechanical deformation undergone by the piezoelectric actuators 7a, 7b, following upon application of a potential to their floating electrodes, into a linear displacement of the read/write transducer 5. To a first approximation, apart from hysteresis phenomena due to the non-unique relation between the potential applied to the floating electrodes of the piezoelectric actuators 7a, 7b and the mechanical deformation undergone by the piezoelectric actuators, when positive potentials are applied to the floating electrodes a displacement of the read/write transducer 5 is obtained in a first direction, whilst when negative potentials are applied to the floating electrodes, a displacement of the read/write transducer is obtained in a second direction opposite to the former one.
For this purpose, the piezoelectric actuators 7a, 7b are driven by power-amplifier circuits having output dynamics both positive and negative with respect to the ground of the actuator. Typically, power-amplifier circuits are used having output dynamics of several tens of volts (for example, from xc2x112 V to over xc2x140 V).
The power-amplifier circuits are controlled by a control logic unit able to perform all the control algorithms required for displacing the readwrite transducer on all the tracks of the hard disk according to the data-reading requirements.
Since these algorithms are altogether in numerical form (i.e., they are numerical processings that are performed by calculating machines, such as microprocessors operating on numbers in binary form electronically represented by digital signals), whereas the control signals to be supplied to the power amplifiers are of an analog type (typically a voltage or an electrical charge), an interface circuit must be provided between the control logic unit and the power-amplifier circuits.
At present, the interface circuit and the power-amplifier circuits are formed in two distinct chips of semiconductor material. This involves a reduced reliability and poor immunity to external disturbance, in that the interconnections between the interface circuit and the power-amplifier circuits are made on the printed circuit to which the two chips are welded, and are thus affected by the noise produced by the other power circuits present on the printed circuit itself (for example, circuits for driving the induction motor and the motor for rotating the hard disk). In addition, there is also a reduction in the speed for controlling the piezoelectric actuators due to the delays that the control signals, supplied by the control logic unit, undergo along the path that takes them to the power-amplifier circuits. Furthermore, the presence of two distinct chips results in a non-indifferent increase in the area of printed circuit required.
An embodiment of the present invention provides a driving circuit for piezoelectric actuators, in particular for a readwrite transducer for hard disks, that will overcome the limitations pointed out with reference to the prior art. The driving circuit is integrated in a chip of semiconductor material and includes an interface circuit having an input that receives at least one control signal generated by a control logic unit; and a power circuit that drives the piezoelectric actuators and has an input directly connected to said interface circuit.